[now historical] An old-style UUCP electronic-mail address
specifying hops to get from some assumed-reachable location to the
addressee, so called because each hop is signified
by a bang sign. Thus, for example, the path
...!bigsite!foovax!barbox!me
directs people to route their mail to machine bigsite (presumably a well-known location
accessible to everybody) and from there through the machine foovax to the account of user me on barbox.

In the bad old days of not so long ago, before autorouting mailers
and Internet became commonplace, people often published compound bang
addresses using the { } convention (see glob) to
give paths from several big machines, in the hopes
that one's correspondent might be able to get mail to one of them reliably
(example: ...!{seismo, ut-sally,
ihnp4!rice!beta!gamma!me}). Bang paths of 8 to 10 hops were
not uncommon. Late-night dial-up UUCP links would cause week-long
transmission times. Bang paths were often selected by both transmission
time and reliability, as messages would not infrequently get lost. See
the network and
sitename.